How much current is drawn by an LED night light which runs off the mains?
BTW what sort of circuit is used for a LED to run off the mains. Is it just a drop down resistor and a rectifier?
-- (The one I have also has a photocell to switch off the light in the day but I don\'t think that bit of the circuit affects the current drawn.)
A web search engine would help. It shows the information right on your computation machine :-)
This one is 250mW:
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I am not saying this is the proper way to do it. Don't sue me if someone get shocked. A single 100K resistor will light up an LED at
10mA. LED is a diode.
The author there makes an interesting comment.
"... after only three months, the light gradually faded until it was virtually useless. This has happened to me several times before with other lights I have tried and results from the use of cheap inferior white LEDs, which have phosphors that quickly fade."
I had no idea people were using phosphors that would be significantly damaged by blue LED emissions. I am trying to understand exactly how that would happen, besides. So I guess the author must mean that the phosphors are simply chemically unstable to begin with.
Does anyone happen to know precisely which phosphor material is being used in these "cheap inferior white LEDs," as the author wrote? I'd like to know.
Jon
On a sunny day (Wed, 23 Sep 2009 21:06:53 +0100) it happened Peter wrote in :
I have one, it says 1W. Pretty bright though, it says 'Deco LED', and is made by Philips, maybe not really a night light.
No, I think it could be some switcher. Could be a series capacitor, series resistor, with the LED in a bridge too. It is all in the bulb's base, so cannot see it. If it ever breaks down I will look.
That's close to a 10W incandescent bulb. Most light nights are 4 to
5W.
o.
It can be any voltage. But for safety reason, usually step down to
12V. I got sample light strips with 75 ohms resistors and 3 1/4W LEDs in series. They are safe to be touched by small kids.
Looks like a Democrat circuit... defined as "green", but actually low efficiency and low light output ;-) ...Jim Thompson
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LOSE the WUSS
BRING BACK BUSH
I suppose it depends how cheap and nasty your white LED supplier is. I have had a white LED based night light running for best part of 5 years and the amazing thing is that the minimum current required for a dark adapted eye to see by is around 10uA. So in principle you could just run a night lite off a capacitive divider and rectifier with a suitable choke/resistor in series to protect the diode from any nasty transients that come down the line. Power used by that at 3mW would be hard for a normal mains electricity meter to detect.
Mine uses a 9v rechargable that lasts about a year in its standby state
- it will run as a torch if switched on. But it is never quite off which makes the torch a lot easier to find if the power drops after dark.
It is a surprising claim. I expect there are some yellow organic dyes or pigments like rubrene that do degrade fairly quickly under the influence of blue to near UV light. But I'd be very surprised if the normal inorganic phosphors of white LEDs ever failed in that way.
Question now is which brands of white LED are using unstable phosphors (if any).
Regards, Martin Brown
Hold an AM radio next to it, then you'll know.
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John Larkin once said that everything electronic that emits something such as light doesn't live forever. On every project I was involved in with serious LEDs on there lifetime was one of the agenda items to discuss.
I assume it's like with CFL. Our first round died within the year, well short of their 10x light bulb claims. So I gave up on this technology for many years. Then I tried again, this time Philips Marathon. They seem to last forever.
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I saw one of those whiny "green" shows on "edjamacational" TeeVee, and they said, "Don't throw incandescents in the trash. The glass takes hundreds of years to break down."
What a bunch of idiots. Glass is ALREADY DIRT!!
Cheers! Rich
Speaking of fading lights, I have a Totaline automatic switchover thermostat in my home. IIRC, it uses an EL backlit display but it no longer backlights. It used to be quite bright; the backlighting was very visible even with room lights on. Now, after about 5 years, the backlighting is totally gone. I can still read the display, but the room lights have to be on. Wondering... would an LED restore the backlighting? Where would it need to be positioned... directly behind the display or to the side?
-- Dave M masondg44 at comcast dot net
They should still emit a bunch of blue.
I bought a bunch of cool blue led night lights, a year or so ago, and set one aside as a reference to check against once in a while. No visible diff so far. Must have got lucky.
Neons usually last a few years. ELs fade and do other weird things.
If they're left on all the time, they last a lot more than if they're cycled. 7 watts isn't a lot of power to leave on.
John
Blue for a night light? Yuck.
We still use the little incandescent ones. They last several years and have a nice spectrum.
I think they have 14W or so. But they are way too bright as a night light. Also, these don't seem to mind cycling and their brightness is usable right after turn-on (except in winter). The CFL flood light outdoors, different thing. Easily takes a minute to get from near-IR to white. Or several minutes in winter.
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Well, I didn't want to be as vague as all that. I work with rare earth phosphors, regularly. And they are pretty hardy materials, fired usually at highish temperatures and not prone to changing structure by the fly-speck energies found in near-400nm photon. I mean, this is only 3eV or so. Most I've worked with are fine at 5eV almost forever.
I find Martin's response just about exactly how I feel about it. There are organic/laser dyes that do degrade. But these aren't what I thought they were using in white LEDs. As he points out in what I consider to be broad agreement with me, rare earth ceramic/inorganic phosphors pretty much just work all day long.
I think the LED itself can indeed fail. But not the phosphor. That's hard for me to imagine, right now. In the case you talk about with CFLs, I've opened up just about every CFL that has ever failed in my home and taken a close look -- dismantling every single part, in fact. There is a particular brand I find to have very dangerous designs (it is sold at Costco) and those burn up for just about every imaginable reason. I've had capacitors explode and burn, transistors literally blow their sides out, inductors which overheated and caught fire, etc. I think they under-design the entire thing. It's all at risk. On the other hand, as you say, the Marathon units are much better designed. hehe. In fact, they also use a more complex method to actually attach the circuit board between the screw end and the bulb that makes it harder for me to gain access, too. But when they fail, it seems to be that the parts do NOT burn up. (I've only had two fail so far from them and I think both cases related to the fluorescent bulb itself -- but NOT the failure of its phosphor material.)
Jon
Now that's how I used to "listen" to my software operating on the Altair 8800! Wonderful way to detect loops, etc.
Jon
I actually bought those at Costco, at the time when a utility/state subsidy made them irresistably cheap. A Dollar a piece.
The ones from the previous run failed rather quietly. Sometimes I also suspected the electronics but some had a distinctively blackened fluorescent coil. Those were the ones that didn't fail point-blank but would have a harder and harder time to start, or would only start on warmer days.
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We did that on programmable pocket calculators. When the warbling and chirping changed to an even buzz that meant the routine had finished :-)
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Better(*) two LEDs in antiparallel. Reverse voltage in the 'off' state fries a single LED.
(*) Of course, there are even better ways :)
Pere
Blue looks subjectively a lot dimmer to the eye even though the photon energy is higher. But having said that I suspect what causes the LEDs in this circuit to fade is damage inflicted by fast mains transients and inductive spikes on the mains.
Compared to incandescent filament bulbs they are fabulously reliable - and in many modern display applications have excellent collimation built in. Signs on motorways that are highly visible from a long way off but do not dazzle nearby drivers are one major application.
I can't recall ever seeing a neon indicator that has failed without being smashed. I have some lying around from decades ago that still work. Laser tubes seem to expire more easily.
Depends how many lots of 7W you have left on.
Regards, Martin Brown
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